THURSDAY, March 1 (HealthDay News) --An estimated 4 million trees are disappearing from urban areas in the United States each year, a new study found.

A little less than 1 percent of urban tree cover is being lost annually.

Researchers from the U.S. Forest Service used satellite imagery to analyze the amount of tree cover compared to the amount of land covered by pavement, rooftops, grass or bare soil in 20 U.S. cities. They found tree cover declined in 17 of the cities, most markedly in New Orleans, Houston and Albuquerque, N.M. (New Orleans's tree loss, however, is most likely due to the devastation of Hurricane Katrina in 2005, the researchers noted.)

"Trees are an important part of the urban landscape," Michael Rains, director of the Forest Service's Northern Research Station, said in a Forest Service news release. "They play a role in improving air and water quality and provide so many environmental and social benefits."

Trees also have an economic benefit: helping reduce heating and cooling costs.

"Our urban forests are under stress, and it will take all of us working together to improve the health of these crucial green spaces," U.S. Forest Service Chief Tom Tidwell said in the news release.

The researchers encouraged more tree-planting efforts to help stop the loss.

"Tree cover loss would be higher if not for the tree-planting efforts cities have undertaken in the past several years," said the Forest Service's David Nowak, who authored the study with his colleague, Eric Greenfield. "Tree planting campaigns are helping to increase, or at least reduce the loss of, urban tree cover, but reversing the trend may demand more widespread, comprehensive and integrated programs that focus on sustaining overall tree canopy."

The study, which was published in Urban Forestry & Urban Greening, showed that 16 cities had an increase in "impervious" cover, that is, pavement and rooftops. The cities with the greatest increase in impervious cover were Los Angeles, Houston and Albuquerque.

Of the 20 cities studied, Atlanta had the most tree cover (54 percent), while Denver had the least (10 percent). New York City led all cities in impervious cover (61 percent), while Nashville had the least (18 percent).

Syracuse, N.Y., was the only city in the study to show an increase in tree cover. Researchers said, however, the increase was due to the proliferation of European buckthorn, an invasive shrub from Europe.